Recently, with the development of computer techniques, techniques related to a device that is incorporated in a computer and a peripheral device that is connected externally to a computer are rapidly progressed. In these techniques, the development of an information storage apparatus represented by a Hard Disk Drive (HDD) and a storage medium (storage reproducing medium) in which information is stored is especially active and various types of information storage apparatuses and storage media are currently on the market. As methods of storing information, a method of storing information by a combination of magnetization directions of plural pieces of magnetization under a rule of expressing 1 bit of information in a magnetization direction of one magnetization (parallel or antiparallel with respect to a predetermined direction) is conventionally widely known. In this method, in a storage area provided in a storage medium, information is recorded by forming magnetization in a direction according to an electric signal for recording (recording signal) representing information, and the information is reproduced by reading the direction of magnetization to generate an electric signal for reproduction (reproducing signal) representing the direction of magnetization. Therefore, in an information storage apparatus that employs this method, there are provided a recording device to form magnetization in a direction according to a recording signal and a reproducing device to generate a reproducing signal to represent the direction of magnetization.
Recently, a recoding density in a storage area becomes denser and a small reproducing device appropriate for reading information in such a high recording density is strongly desired. As a reproducing device appropriate for downsizing, there is known a reproducing device to read a direction of magnetization by utilizing a film called a magnetoresistive effect film. This magnetoresistive effect film has a layered structure sandwiching a nonmagnetic layer between two ferromagnetic layers. The magnetoresistive effect film has a property (magnetoresistive effect) that a magnitude of an electrical resistance to a current flowing between the two ferromagnetic layers via the nonmagnetic layer is different according to whether magnetization directions of the two ferromagnetic layers are in the parallel state to each other or in the antiparallel state to each other. The reproducing device that utilizes the magnetoresistive effect film electrically detects a change of an electrical resistance value between the two ferromagnetic layers according to a magnetization direction of each magnetization to read the magnetization direction. The method that utilizes the magnetoresistive effect has a simple mechanism of detecting a magnetization direction in comparison with a method that utilizes electromagnetic induction by a coil, so that downsizing of the reproducing device is achieved.
FIG. 1 is an external view illustrating surroundings of a reproducing device, in a magnetic head of a Hard Disk Device (HDD) that employs the reproducing device having a magnetoresistive effect film.
In FIG. 1, a reproducing device 104 is illustrated in a xyz-orthogonal coordinate system defined such that a surface of a magnetic disk 103 that rotates while closely approaching to the reproducing device 104 is set as a xy-plane, a direction of normal to the xy-plane is set as a z-axis, and a position of the reproducing device 104 is set as the origin. Here, the x-axis direction is a direction in which the reproducing device 104 moves relatively to the magnetic disk 103 and it is a circumference direction of the magnetic disk 103. The reproducing device 104 moves relatively to the direction of the x-axis, thereby sequentially approaching to 1 bit areas which are unit of storage areas aligned along each track (a circumference area with a constant radius) of the magnetic disk 103 and each of which has one magnetization representing 1 bit of information, and reads information. Here, each 1 bit area has a magnetization oriented in either a positive direction or a negative direction of the z-axis in FIG. 1.
As illustrated in FIG. 1, the reproducing device 104 includes two magnetic shield layers 100, 101 and a magnetoresistive effect film 102 arranged between these two magnetic shield layers 100, 101. Incidentally, as a constituent device of the reproducing device 104, in addition to the above-described magnetic shield layers 100, 101 and the magnetoresistive effect film 102, there are also a substrate serving as a supporting member to support them and a magnetic domain control film disposed between the magnetic shield layers 100, 101, and whose illustration is omitted here. In the reproducing device 104, an external magnetic field other than the magnetic field of the 1 bit area that most closely approaches to the magnetoresistive effect film 102 and that is targeted for reading the magnetization direction is shielded by the magnetic shield layers 100, 101. That is, only the magnetic field from the most closely approaching 1 bit area is detected by the magnetoresistive effect film 102. Here, the above-described nonmagnetic layer and the two ferromagnetic layers are built up in the magnetoresistive effect film 102 in the x-axis direction of FIG. 1. Next, a layer structure of the magnetoresistive effect film 102 will be explained.
FIG. 2 is a schematic diagram illustrating a layer structure of the magnetic effect film in FIG. 1.
The magnetoresistive effect film 102 has a layer structure such that an auxiliary underlayer 1c, an underlayer 2, an antiferromagnetic layer 3, a first ferromagnetic layer 4, a nonmagnetic layer 7, a second ferromagnetic layer 8, a surface protective layer are built up in this order on the magnetic shield layer 100 on the left side of FIG. 1. Here, the antiferromagnetic layer 3 has an antiferromagnetic property, the first ferromagnetic layer 4 and the second ferromagnetic layer 8 have a ferromagnetic property, and the nonmagnetic layer 7 is formed of a material whose magnetization ratio is remarkably small and hard to be magnetized. Further, the auxiliary underlayer 1c serves to increase adhesiveness between the underlayer 2 and the magnetic shield layer 100 of FIG. 1, and the underlayer 2 serves to fix the antiferromagnetic layer 3 to be smooth on the magnetic shield layer 100 on which the auxiliary underlayer 1c is built up. The surface protective layer 9 serves to protect a surface of the magnetoresistive effect film 102, and also contacts the magnetic shield layer 101 on the right side of FIG. 1.
In a state where a ferromagnetic layer and an antiferromagnetic layer contact with each other, as the first ferromagnetic layer 4 and the antiferromagnetic layer 3 illustrated in FIG. 2, it is known that, due to exchange coupling (a kind of magnetic interaction and it is an interaction between magnetizations) generated in a boundary surface between the ferromagnetic layer and the antiferromagnetic layer, a magnetization direction of the ferromagnetic layer is fixed so that the magnetization direction is still hard to be changed even receiving an influence of external magnetic field. FIG. 2 illustrates a state in which the magnetization direction of the first ferromagnetic layer 4 bordering the antiferromagnetic layer 3 is fixed in a direction of a rightward arrow in the diagram as an example. On the other hand, the second ferromagnetic layer 8 is away from the antiferromagnetic layer 3, so that the second ferromagnetic layer 8 is not such restricted as the first ferromagnetic layer 4 and the magnetization direction of the second ferromagnetic layer 8 is affected by the external magnetic fields and changed. Specifically, being affected by the magnetic field of 1 bit area approaching to the magnetoresistive effect film 102, the magnetization direction of the second ferromagnetic layer 8 in the magnetoresistive effect film 102 is changed. Here, due to the magnetoresistive effect, a magnitude of a resistance that a current flowing between the first ferromagnetic layer 4 and the second ferromagnetic layer 8 receives becomes small when the magnetization of the first ferromagnetic layer 4 and the magnetization of the second ferromagnetic layer 8 are in a state in which they are parallel to each other, and becomes large when the magnetization of the first ferromagnetic layer 4 and the magnetization of the second ferromagnetic layer 8 are in a state in which they are antiparallel to each other. By utilizing such property, the reproducing device 104 of FIG. 1 outputs a detection signal of the current flowing between the first ferromagnetic layer 4 and the second ferromagnetic layer 8 when a voltage is applied between these two ferromagnetic layers. By detecting a change in the current represented by this detection signal, the magnetization direction of each 1 bit area is detected.
In a read method that utilizes the magnetoresistive effect, in order that the magnetoresistive effect film 102 receives only an influence of a the magnetic field of a 1 bit area approaching to the magnetoresistive effect film 102, it is desired that a length of the magnetoresistive effect film 102 in the x-axis direction (read gap length “Lb” in FIG. 1) is approximately equal to or less than the length of the 1 bit area in the x-axis direction (bit length “Lb” in FIG. 1). Here, in order to improve a recording density of a magnetic disk, it is desirable to shorten the bit length, and therefore, in order to achieve a high recording density, it is desired that a reproducing device having a short read gap length that corresponds to the short bit length is obtained. Here, it is known that, in order to generate the exchange coupling enough for fixing the magnetization of the first ferromagnetic layer 4 between the antiferromagnetic layer 3 and the first ferromagnetic layer 4 contacting the antiferromagnetic layer 3, the thickness of the antiferromagnetic layer 3 is desired to be larger than a certain degree. However, if the antiferromagnetic layer 3 is thickened, the read gap length Lrg of FIG. 1 also increases, so that it is hard to achieve a high recording density. Specifically, it has been an obstacle to achieve a high recording density that earlier than year 2003, as a material of the antiferromagnetic layer 3, an alloy of platinum (Pt) and manganese (Mn) or an alloy of lead (Pd), platinum (Pt) and manganese (Mn) are often employed. With these materials, the antiferromagnetic layer 3 is desired to have a layer thickness of 15 nm or more to generate sufficient exchange coupling for the first ferromagnetic layer 4 in the thickness of only several nm. Recently, there is reported that (see, for example, Japanese Laid-Open Patent Publication No. 2005-244254), if an alloy (hereafter abbreviated as Ir—Mn) of iridium (Ir) and manganese (Mn) is employed as a material of the antiferromagnetic layer 3, sufficient exchange coupling is generated in the antiferromagnetic layer 3 whose layer thickness is half or less (namely, 7.5 nm or less). In this way, by employing Ir—Mn as a material of the antiferromagnetic layer 3, it is possible to obtain a small reproducing device suitable for a high recording density.
As described above, the underlayer 2 serves to fix the antiferromagnetic layer 3 to be smooth on the magnetic shield layer 100 on which the auxiliary underlayer 1c is built up, and, as a material of the underlayer 2, an alloy formed by combining some of nickel (Ni), iron (Fe) and chrome (Cr), and a metal such as copper (Cu), ruthenium (Ru) and tantalum (Ta) have been conventionally used, also in Japanese Laid-Open Patent Publication No. 2005-244254.
However, if the antiferromagnetic layer 3 employing Ir—Mn is built up on the underlayer 2 formed of these materials, Ir—Mn is oriented in a [111] plane on the underlayer 2, so that a smooth film is not obtained. If the first ferromagnetic layer 4, the nonmagnetic layer 7 and the second ferromagnetic layer 8 are built up on the antiferromagnetic layer 3 whose surface is uneven, these built up layers also become layers lacking smoothness, and specifically because the nonmagnetic layer 7 is thin as compared to other layers, the nonmagnetic layer 7 is affected by the unevenness on a boundary surface of the antiferromagnetic layer 3 and tends to be a layer in a shape curling like a wave. If the nonmagnetic layer 7 becomes such a curled shape, the magnetic field of the first ferromagnetic layer 4 near the boundary surface of the nonmagnetic layer 7 affects a magnetization state of the second ferromagnetic layer 8, so that the responsiveness of the magnetization of the second ferromagnetic layer 8 to the magnetic field of the 1 bit area as the reading target is lowered. This interaction acting between the two ferromagnetic layers via the nonmagnetic layer is generally called Orange Peel effect. Furthermore, if the nonmagnetic layer 7 becomes a curled shape, when a voltage is applied between the first ferromagnetic layer 4 and the second ferromagnetic layer 8 and a current flows through, due to the decline of tolerance for the voltage, a breakage of the nonmagnetic layer 7 may occur. In this way, in a state where the responsiveness to an external magnetic field is low and the tolerance to the voltage is also low, the ability as the reproducing device is not fully exerted, and poses a problem.
When Ir—Mn is employed as a material of the antiferromagnetic layer in the magnetoresistive effect film, although there is an advantage that downsizing of a reproducing device is achieved, in order to utilize this advantage, a devise is desired to suppress that the antiferromagnetic layer becomes low in the smoothness.
Incidentally, in the above description, the reproducing device 104 that utilizes the magnetoresistive effect film 102 is described as a way of example. However, the above-described problem when Ir—Mn is employed as the material of the antiferromagnetic layer may also occur in a magnetoresistive effect device in general to detect a magnetization by utilizing the magnetoresistive effect film 102, other than the reproducing device. For example, this problem may also occur in a random access memory (RAM) that includes the plural magnetoresistive effect film 102 of FIG. 2 and stores information in form of magnetization directions of the plural second ferromagnetic layers 8.